Identification of a Defective Filament in a Fluorescent Lamp

ABSTRACT

Control of delivery of current through one or more discharge lamps. Methods include alternately switching on and off switching elements that control a fluorescent lamp, in response to receiving input, until the brightness of the lamp decreases to a threshold. Further, methods include providing control signals at complementary duty cycles to further decrease the brightness and alternating the duty cycles of the signals applied to the filaments of the fluorescent lamp.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/125,897, filed May 22, 2008, which claims priority under 35 U.S.C.§119(e) to U.S. Provisional Application No. 60/939,469 filed May 22,2007. The disclosures of the applications referenced above areincorporated herein by reference.

BACKGROUND

The subject matter of the present specification relates to controllersto operate discharge lamps, e.g., fluorescent lamps.

In general, a fluorescent lamp assembly is made from a tube filled withan inert gas, for example, argon, and some low pressure mercury vapors.The inside of the tube is coated with a fluorescent material. Thefluorescent lamp includes two electrodes attached to either end of thetube. The electrode includes a filament surrounded by an emittingcathode. When the filament is warmed up, the cathode emits thermalelectrons that form an electron cloud around the cathode. When apotential difference is applied between the filaments, the chargecarriers accelerate towards the positive electrode. While migratingtowards the positive electrode, the charge carriers collide with one ormore mercury atoms. If the energy with which a charge carrier collideswith a mercury atom is sufficiently high, then the mercury atom emitsultraviolet radiation. When the coating on the inside of the tubereceives the ultraviolet radiation, the coating emits radiation in thevisible spectrum, which appears as light.

Electronic ballasts are typically used to operate fluorescent lamps,such operations including switching the lamps on and off, dimming thelamps, and the like. An electronic ballast for fluorescent lamp dimmingcontrol can use several circuits, for example, a series LC resonantseries loaded circuit, a series resonance parallel loaded circuit, aseries parallel resonance circuit, and the like. The series circuits aretypically controlled by either frequency or the duty cycle ofsymmetrically chopped input voltage pulses. A series resonance parallelloaded circuit, used in an electronic ballast, behaves like a low-passfilter and shows a high gain at high impedance that is required by thefluorescent lamp during ignition and low dimming. The input of theelectronic ballast comes from a source, for example, a DC source. Twoswitching elements turn on and off in response to a signal from acontroller to convert the DC voltage into an AC voltage. The controllercontrols states of the switching elements, and thus the waveform of theAC voltage, in accordance with a desired dimming level. By controllingthe timing of turning the switching elements on and off, the currentflowing through a fluorescent lamp can be changed, and the light outputof the fluorescent lamp can be varied. An electronic ballast operatedusing a series resonance parallel loaded circuit that includes doubleswitch choppers at the DC output is generally adjustable, providing thehigh voltage required for lamp ignition, short circuit proof, and offersincreased voltage in high impedance and low load during dimming.

SUMMARY

This specification describes technologies relating to control ofdelivery of current through one or more discharge lamps(e.g.,fluorescent lamps).

In one aspect, a method to identify a defective filament in afluorescent lamp in a circuit is described. The method includessupplying a voltage to a circuit including one or more fluorescent lampsand an anti-parallel transformer. Each fluorescent lamp includes twofilaments. A primary side of the transformer is operatively coupled to afirst filament and a secondary side of the transformer is operativelycoupled to a second filament. The first filament and the second filamentdraw a first current flowing through the primary side of the transformerand a second current flowing through the secondary side of thetransformer, respectively. The method includes monitoring a resistanceof the transformer. The resistance is below a threshold as long as thefirst filament and the second filament are present in the circuit whichcauses the first current and the second current, respectively, to flowthrough the primary side and the secondary side of the transformer,respectively. Upon determining that the resistance increases above thethreshold, the method includes identifying that either the firstfilament or the second filament is absent from the circuit.

This, and other aspects can include one or more of the followingfeatures. The first filament and the second filament can be included ina same fluorescent lamp. The first filament can be determined to beabsent from the circuit when current does not flow through the primaryside. The voltage can include an operating voltage supplied at anoperating frequency. The method can further include, upon identifyingthat either the first filament or the second filament is absent from thecircuit, supplying a detection voltage at a frequency greater than theoperating frequency to the circuit. The detection voltage can bealternately turned on for a first duration and turned off for a secondduration. A resistance of the transformer can be when the detectionvoltage is supplied to the circuit. The method can further include, upondetecting that the measured resistance is less than the threshold,turning off the detection voltage and supplying the operating voltage atthe operating frequency. The first duration can be 10 ms and the secondduration can be 300 ms. Other aspects include a processor configured toperform the operations described herein.

In another aspect, a system to identify a defective filament in afluorescent lamp in a circuit is described. The system includes acircuit including a first fluorescent lamp, a second fluorescent lamp, atransformer including a primary side operatively coupled to a firstfilament of the first fluorescent lamp and a secondary side of thetransformer operatively coupled to a second filament of the secondfluorescent lamp, a voltage source to supply a voltage, and a processoroperatively coupled to the first driver and the second driver,configured to perform operations described herein.

The described systems and techniques can result in one or more thefollowing advantages. The brightness of fluorescent lamps in a lightingsystem can be decreased using switching elements that control thefluorescent lamps. Alternate switching of the complementary duty cyclesof the switching elements that control the fluorescent lamps can ensurethat one filament does not fail earlier than the other filament.Coupling two filaments (from a same fluorescent tube, or from twoseparate fluorescent tubes—e.g., two adjacent filaments in anapplication in which two fluorescent tubes are operated in parallel)respectively to two sides of an anti-parallel transformer and monitoringan impedance offered by the transformer can enable detecting whether oneof the filaments is absent or defective. This method can be extended toall fluorescent lamps in the lighting system. The same processor can beused in different lighting systems. The fluorescent lamps in onelighting system need not share the same specification with those inanother lighting system. The processor can be configured to detect thetype of fluorescent lamps in a lighting system and to provide theappropriate parameter set including parameters to operate thefluorescent lamps. In this sense, the processor is portable withinlighting systems. The processor can control multiple fluorescent lampswithin the same lighting system, and can be configured to enable atechnician to determine which control signal, the supply of which iscontrolled by the processor, is supplied to which fluorescent lamp inthe lighting system. The processor can also be configured to performdigital operations on analog voltage and current signals to identifycurrent drawn by a fluorescent lamp, and distinguish the lamp currentfrom current generated due to parasitic capacitance. This isparticularly useful when the lamp is dimmed because the magnitude ofcontribution of the parasitic capacitance at low lamp brightness levelscan be comparable to the magnitude of the lamp current itself. In suchscenarios, distinguishing between lamp current and parasitic capacitancecontributions enables better control of lamp dimming. The systems andtechniques described herein can also enable fine control of the outputpower of fluorescent lamps when the lamps are dimmed. Such fine controlcan result in maximizing the life of the fluorescent lamp. Faultdetection and protection, and in-circuit replacement of the fluorescentlamps are also possible. The processor can perform these digitaloperations for the phase determination using a single analog to digitalconverter (ADC).

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features,aspects, and advantages may become apparent from the description, thedrawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, which includes FIGS. 1A, 1B and 1C, is example circuitry of alighting system.

FIG. 2 is a flowchart of an example process to control a fluorescentlamp.

FIG. 3 is a schematic of an example circuit to control two fluorescentlamps.

FIG. 4 is a flowchart of an example process to identify a defectivefilament in a circuit including fluorescent lamps.

FIG. 5 is a schematic of an example circuit to identify a defectivefilament in a fluorescent lamp.

FIG. 6 is a flowchart of an example process to identify a fluorescentlamp.

FIG. 7 is a schematic of an example circuit including a processor toidentify parameters to operate a fluorescent lamp.

FIG. 8 is a flowchart of an example process to identify a current drawnby a fluorescent lamp in a circuit.

FIG. 9 is a schematic of an example circuit to identify a current drawnby a fluorescent lamp in a circuit.

FIG. 10 is a flowchart of an example process to identify a fluorescentlamp among multiple fluorescent lamps.

FIG. 11 is a schematic of an example circuit including multiplefluorescent lamps receiving control signals.

FIG. 12 is a schematic of an example processor to control an industriallighting system.

DETAILED DESCRIPTION

The systems and techniques described herein can be implemented in one ormore devices, for example, one or more integrated circuit (IC) devices,finite state machine (FSM) processors, and the like, that areoperatively coupled to circuits to operate lighting systems, forexample, industrial lighting systems in buildings. A processor canreceive control signals, for example, voltage signals, from one or morepower sources, and transmit the control signals to one or morecomponents of an industrial lighting system.

FIG. 1, which includes FIGS. 1A, 1B and 1C, depicts a schematic ofexample circuitry of a lighting system 1000 including electricalcomponents, for example, switching elements, transformers,inductor-capacitor (LC) and resistor-capacitor (RC) components, and thelike, operatively coupled in one or more circuits to operate dischargelamps, for example, fluorescent lamps. Lighting systems, such aslighting system 1000, are typically installed in buildings and arefrequently operated using ballast controllers operatively coupled to thelighting systems.

A ballast controller includes a processor configured to provide voltagepulses to the lighting system 1000 to enable a technician to control thelamps in the lighting system 1000. In some implementations, the lightingsystem 1000 includes a series resonant parallel loaded ballast withdouble switching elements. To dim a fluorescent lamp in the lightingsystem 1000, the processor operates the two switching elementssimultaneously causing a change in the current drawn by the fluorescentlamps. In addition to controlling the operation of the fluorescentlamps, the processor in the ballast controller is configured to performmultiple operations that will be described in reference to the figuresthat follow.

FIG. 2 is a flowchart of an example process 100 to control a fluorescentlamp. The process detects input to decrease a brightness of afluorescent lamp (105). The fluorescent lamp is included in a lightingsystem, for example, the lighting system 1000 shown in FIG. 1, and isoperated by a driving signal. In some implementations, the fluorescentlamp can be operatively coupled to a series resonance parallel loadedcircuit that receives voltage from a source controlled by a processor.The processor is a component of a ballast controller to operate thelighting system. The circuit can include two switching elements thatreceive control signals, for example, voltage pulses, from a processorto operate the fluorescent lamp.

In response to detecting the input, the process 100 operates the drivingsignal using two alternately applied duty cycles that are substantiallycomplementary to each other (110). For example, the driving signal canbe supplied to the fluorescent lamp via two switching elements, each ofwhich is controlled by a corresponding control signal. The processor canchange the current to the fluorescent lamp by controlling the switchingelements. In some implementations, the first and second control signalscontrolled by the two switching elements can be applied at the first andsecond duty cycles, respectively, resulting in the two duty cycles beingapplied to the driving signal. In addition, the processor can alternatethe application of the first duty cycle and the second duty cycle to thefirst control signal and the second control signal, respectively.Further, the processor can increase a frequency at which the duty cyclesare applied. These operations on the switching elements cause a decreasein a brightness of the lamp.

The process simultaneously increases the first duty cycle and decreasesthe second duty cycle while substantially maintaining a complementarityof the first duty cycle and the second duty cycle (115). In someimplementations, the decrease in brightness can be measured by afeedback circuit included in the lighting system. For example, thebrightness of the lamp can be measured by converting brightness into anelectrical signal, for example, a digital signal, that is associatedwith a value. The measured value can be compared with a stored valueassociated with a brightness threshold. If a difference between ameasured and stored value is within a predetermined limit, then it isdetermined that the brightness of the lamp has reached a threshold.

To further reduce the brightness of the lamp, the first and duty cyclescan be simultaneously increased and decreased. For example, the firstduty cycle and the second duty cycle can be 50% each. The two dutycycles are considered substantially complementary as long as a summationof the duty cycles is 100% or at least 90%. When the lamp brightnessdecreases to a threshold, for example, 20% of the operating brightness,the first duty cycle can be increased to more than 50%, for example, 70%and the second duty cycle can be decreased to 30%. This maintains thecomplementarity of the two duty cycles because the summation is 100%.Although the example process 100 describes the summation of the two dutycycles to be 100%, in some implementations a delay is intentionallyintroduced in the operation of the switching elements to ensure that thetwo switching elements never conduct simultaneously. Therefore, asummation of the first and second duty cycles is substantially 100%because of the delay.

Below a brightness level corresponding to a first threshold, forexample, 20% of the output power, the first and second duty cycles canbe held at fixed values. For example, when the brightness levelcorresponds to approximately 80% of the output power, the first andsecond duty cycles applied to the driving signal are 50%. As thebrightness decreases from 80% to the first threshold, for example, 20%,the first duty cycle can be increased in steps while simultaneouslydecreasing the second duty cycle. In some scenarios, the first dutycycle can be increased to 51%, 52%, 53%, and so on, while the secondduty cycle can simultaneously be decreased to 49%, 48%, 47%, and so on.This can be repeated until the brightness level corresponds to the firstthreshold. In addition, the duty cycles can be alternately applied.

At the first threshold, the first and second duty cycle can be, forexample, 70% and 30%, respectively. Continuing to simultaneouslyincrease and decrease the first and second duty cycle can cause lampflickering or extinguishing. It can also lead to failure of the lampelectrodes. To avoid these, the process maintains first and second dutycycles at these fixed values when the brightness is decreased below thefirst threshold (120). Details related to operating a fluorescent lampusing complementary duty cycles can be found in U.S. patent applicationSer. No. 10/883,342 entitled “Mixed Mode Control for DimmableFluorescent Lamp,” which was filed on Jul. 1, 2004, the entire contentsof which are incorporated herein by reference. The driving signal can bemodified using pulse width modulation.

FIG. 3 is a schematic of an example circuit 200 to control twofluorescent lamps 205 and 210. The two fluorescent lamps 205 and 210 areoperatively coupled to two switching elements 215 and 220. The switchingelements can be field effect transistors. The circuit 200 includes asource 225 that provides control signals to the first switching element215 and the second switching element 220, thereby providing drivingsignals to operate the two fluorescent lamps 205 and 210. Further, thecircuit 200 is controlled by a processor 230 that controls the voltagesource 225 and performs the operations described with reference to FIG.2. The values to be applied to the first and second duty cycles whilesimultaneously increasing and decreasing the duty cycles can be storedin a memory that is operatively coupled to the processor 230. In someimplementations, the memory can be included in the processor 230.

FIG. 4 is a flowchart of an example process 400 to identify a defectivefilament in a circuit including fluorescent lamps. The process suppliesa voltage to a circuit including one or more fluorescent lamps and ananti-parallel transformer (405). For example, the industrial lightingsystem 1000 can include multiple transformers including theanti-parallel transformer. A first filament and a second filament of afluorescent lamp in the industrial lighting system 1000 can beoperatively coupled to a primary side and a secondary side of theanti-parallel transformer, respectively. Alternatively, a first filamentof a first fluorescent lamp and a second filament of a secondfluorescent lamp can be operatively coupled to a primary side and asecondary side of the anti-parallel transformer, respectively. Eachfilament can serve as a load to the corresponding side of thetransformer, and can draw a current.

Two filaments of one or more fluorescent lamps are typically notidentical to each other, but are similar. When the same control signalis supplied to the two filaments, the temperatures of the two filamentsincrease. While the filament temperatures may not equal each other, thetemperatures will typically differ by a small temperature threshold, forexample, of the order of less than 2° C. Because the current drawn by afilament is a function of filament temperature, the current drawn by onefilament need not be equal to that drawn by the other. Nevertheless, thedifference between the currents drawn by the two filaments can be withina current threshold. As long as the current is within the currentthreshold, an impedance offered by the transformer is small, for exampleon the order of fractions of ohms. If the difference in the currentdrawn by the two filaments increases above the current threshold, thenthe resistance offered by the transformer increases, for example, ashigh as hundreds of ohms. The current drawn by one filament willdecrease in the absence of a filament or in the presence of a defectivefilament.

The process monitors a resistance of the transformer (410). For example,the resistance of the transformer can be measured and associated with avalue. The processor that operates the one or more fluorescent lamps andthe transformer can receive the measured value.

The process checks if the resistance is below a threshold (415). Forexample, a threshold resistance value can be stored in the processor.The processor can compare the measured resistance value with thethreshold value and determine whether the measured resistance is greaterthan or less than the threshold resistance. If the measured resistancevalue is below the threshold, the process continues normal operation(420).

If the measured resistance value is greater than the threshold, theprocess identifies that either the first filament or the second filamentis absent from the circuit (425). The filament may be absent from thecircuit because a fluorescent lamp, of which the filament is acomponent, is defective or because a fluorescent lamp has been removedfrom the circuit. In such scenarios, a current drawn by one filament canbe greater than the current drawn by the other filament, causing theimpedance of the transformer to increase.

Upon identifying that a filament is absent from the circuit, the processturns off the voltage to the lamp (430). For example, when the processordetermines that a filament is absent from the circuit, based on thecomparison of the measured resistance and the threshold resistance, theprocessor can cause the voltage source to stop supplying control signalsto the circuit that includes the one or more fluorescent lamps and thetransformer.

In some implementations, in addition to stopping the supply of controlsignals to the circuit, the processor can provide an output indicatingthat the fluorescent lamp is defective. For example, the processor cancause a light on a display screen to be turned on or an alarm to sound.In response, a technician can examine the fluorescent lamp in thecircuit to identify any defect and replace the fluorescent lamp ifneeded.

When the resistance offered by the transformer is below the threshold,then the processor can supply the control signal, for example, thevoltage signal at an operating frequency. After the voltage signal isturned off, the processor can periodically check the circuit todetermine if the defect in the circuit has been fixed. To do so, theprocessor supplies a detection voltage signal to the one or morefluorescent lamps in the circuit (435). The detection voltage signal canhave a frequency that can be greater than the operating frequency. Ifthe defect in the circuit has been fixed, for example, if the defectivefilament has been identified and replaced, then the resistance offeredby the transformer decreases. If not, then the resistance remainsgreater than the threshold resistance.

The process checks whether the resistance offered by the transformer inresponse to receiving the detection voltage is less than the threshold(440). If the resistance is not below the threshold, then the processretains the lamp in a shutdown state (445). For example, the processorcan periodically receive a measured resistance offered by thetransformer in response to receiving the detection voltage. Theprocessor can compare the measured resistance with a thresholdresistance. In some implementations, the threshold resistance can be thesame as the threshold resistance when the voltage signal is supplied atthe operating frequency. In some implementations, a new thresholdresistance, that corresponds to the detection voltage at the frequencygreater than the operating frequency, can be determined and stored inthe processor. If the processor determines that the measured resistanceis not below the threshold resistance corresponding to the detectionvoltage, then the processor need not cause the voltage source to supplyvoltage signals to the circuit.

Once the defect in the circuit is fixed, for example, the defectivefilament is replaced, and the detection voltage is supplied to thecircuit, the resistance offered by the transformer will be below thethreshold resistance. Upon detecting the measured resistance is belowthe threshold resistance, the processor can turn off the detectionvoltage and can supply voltage at the operating frequency to thecircuit. Further, during periodic testing of the circuit, the processorcan turn on the detection voltage for a first duration, for example, 10ms, and turn off the detection voltage for a second duration, forexample, 300 ms.

FIG. 5 is a schematic of an example circuit 500 to identify a defectivefluorescent lamp. The circuit 500 includes two fluorescent lamps 505 and510. Fluorescent lamp 505 includes two filaments 515 and 520, andfluorescent lamp 510 includes two filaments 525 and 530. The circuit 500includes an anti-parallel transformer 535. The circuit 500 also includesa transformer 550 including one winding on a primary side and twowindings on a secondary side. One of the two secondary side windings canbe operatively coupled to the filament 520 of the fluorescent lamp 505and a primary side 545 of the anti-parallel transformer. The second ofthe two secondary side windings of the transformer 550 can beoperatively coupled to the filament 530 of the fluorescent lamp 510 andthe secondary side 540 of the transformer 545. The primary side of thetransformer 550 can be operatively coupled to a voltage source 555 tosupply voltage signals to the fluorescent lamps and the transformers.The operation of the voltage source 555 can be controlled using aprocessor 560.

The processor 560 is configured to detect the absence of either thefluorescent lamp 505 or the fluorescent lamp 510 by performingoperations described with reference to FIG. 5. In normal operation, thefilament 520 and the filament 530 draw respective currents. In suchscenarios, the anti-parallel transformer 535 offers very low resistance,such resistance being determined based on the current drawn by theresistor. 565 operatively coupled to the circuit 500. If either of thelamps 505 or 510 is absent, for example, because either of the lamps isdefective, then the defective lamp no longer draws a current. Thiscauses the impedance offered by the anti-parallel transformer 535 toincrease. When the resistance increases above a threshold resistancethat is stored, for example, in a memory operatively coupled to theprocessor 560, the voltage supply from the voltage source 555 isterminated.

Although the processor 560 in FIG. 5 is shown to detect defectivefluorescent lamps, the processor 560 can be configured to detect onedefective filament of two filaments in the same fluorescent lamp. Forexample, one filament of the fluorescent lamp can be operatively coupledto the primary side of an anti-parallel transformer, and the otherfilament can be operatively coupled to the secondary side of theanti-parallel transformer. Based on a resistance offered by thetransformer to the flow of current to both filaments, the absence of afilament in the fluorescent lamp can be detected. An industrial lightingsystem can include multiple fluorescent lamps, all of which have thesame specification, and are operable under the same parameters.Different industrial lighting systems can employ fluorescent lamps ofdifferent configurations. A processor can be designed based on the typeof fluorescent lamp to be used in a lighting system. Alternatively, thesame processor can be configured to operate multiple fluorescent lamps.To do so, the parameters for operating each fluorescent lamp can bestored in the processor memory. To use the processor in an industriallighting system, the type of fluorescent lamp that is used in thelighting system needs to be identified, and the parameter setcorresponding to the fluorescent lamp in the system needs to beretrieved from the processor memory.

FIG. 6 is a flowchart of an example process 600 to identify afluorescent lamp. The fluorescent lamp is one of many lamps in anindustrial lighting system. All the lamps in the lighting system havethe same specification and are operable using the same set ofparameters. The parameter set for the fluorescent lamp can be stored ina processor memory as one set of multiple parameter sets correspondingto multiple fluorescent lamps. To retrieve the parameter setcorresponding to the fluorescent lamp, a signal, for example, a voltagesignal, can be transmitted to the processor via a resistor that isoperatively coupled to the processor in a circuit.

The process 600 supplies a voltage to the resistor in a circuit (605).The voltage signal can be supplied as an analog signal or as a digitalpulse. For example, a voltage source can be operatively coupled to theresistor and the processor. The voltage source can supply a pulse to theresistor, in response to which the resistor draws a current.

The process 600 determines a value of the current drawn by the resistor(610). For example, the current drawn by the resistor can be associatedwith a value and can be received by the processor. The resistor that isoperatively coupled with the processor can be chosen based on the typeof fluorescent lamp in the lighting system. For example, a processormanufacturer can decide to associate a current value with each parameterset of a fluorescent lamp that is stored in the processor memory. Insome implementations, the manufacturer can associate a range of currentvalues with each parameter set of a fluorescent lamp. The processormanufacturer can prepare a chart that lists a resistor and acorresponding fluorescent lamp. Based on the chart, a technicianinstalling the industrial lighting system can select a resistor. Tocause the processor to provide the parameter set corresponding to thefluorescent lamps in the lighting system, the technician can couple theresistor with the processor and can provide a voltage signal to theresistor. The processor can receive a value of the current drawn by theselected resistor. In some implementations, the processor can internallygenerate the test current and supply the current to the resistor.

The process identifies a range of current values stored at a storagelocation, within which the determined current value lies (615). Forexample, the processor memory can be a storage location where ranges ofcurrent values are stored along with corresponding fluorescent lamps. Insome implementations, the ranges of current values and the correspondingfluorescent lamps can be stored as look-up tables. The look-up tablescan also store parameter sets that include parameters to operate eachfluorescent lamp. The parameter sets can include one or more of a powerrating of the fluorescent lamp, an operating frequency of thefluorescent lamp, an operating voltage of the fluorescent lamp, and thelike. The identified range of current values can include the currentvalue that was drawn by the resistor and received by the processor.

Based on the received current value, the process retrieves a parameterset corresponding to the range of current values from the storagelocation (620). For example, the processor can retrieve the parameterset that includes the parameters to operate the fluorescent lamp in thelighting system.

The process provides the retrieved parameter set to operate thefluorescent lamp (625). In some implementations, the processor canretrieve all parameters of the parameter set and provides all theparameters to an external device, for example, another processor. Inalternative implementations, the processor can retrieve the parameterset from the look-up tables. As and when the processor receives one ormore signals from the lighting system, the processor can provide theappropriate parameters to operate the fluorescent lamp.

FIG. 7 is a schematic of an example circuit 700 including a processor toidentify parameters to operate a fluorescent lamp. The circuit 700includes a resistor 705 that receives a voltage signal from a source 710to provide control signals, for example, voltage signals. In someimplementations, the resistor 705 can be a 100 kΩ resistor oralternatively, can be a resistor of another rating. In the circuit 700,the source 710 and the resistor 705 are operatively coupled to aprocessor 720 that includes an input receiver 715. In someimplementations, the input receiver 715 can be a soldering joint towhich an end of the resistor 705 can be soldered. In otherimplementations, the input receiver 715 can be configured such that theresistor 705 is inserted into the input receiver 715. When a voltagefrom the source 710 is supplied to the resistor 705, the current drawnby the resistor 705 is received by the input receiver 715. In someimplementations, the input receiver 715 can be included in the processor720. Alternatively, the input receiver 715 can be a separate componentthat is operatively coupled to the processor.

Upon receiving the current drawn by the resistor 705, the processor 720is configured to retrieve a parameter set from a storage location 725,for example, a look-up table, to operate a fluorescent lamp 730according to the operations described with reference to FIG. 6. Althoughthe example circuit 700 shows the storage location 725 within theprocessor 720, the storage location 725 can be located on an externaldevice that is operatively coupled to the processor 720 to accessinformation in the storage location 725. The fluorescent lamp 730 isincluded either in the circuit 700 or in a separate circuit. In someimplementations, the look-up table may not include the parameter set fora new fluorescent lamp. The technician operating the lighting system cancollect new parameters to operate the new fluorescent lamp and store thenew parameter set in the storage location 725. In addition, theprocessor can identify a new resistor and a new current value, andassociate the new current value with the new fluorescent lamp. Also, theprocessor can include a new range of current values in the storagelocation 725, within which the new current value lies. In response toreceiving a current value that lies within the new range of currentvalues, the processor identifies that the lamp that is to be operated isthe new fluorescent lamp, and provides the corresponding new parameterset.

The parameter set to operate a fluorescent lamp can include a voltage tobe supplied to the lamp, and can additionally include a current drawn bythe lamp. In some implementations, the voltage signal can be an analogvoltage signal. In response, the lamp can draw an alternating currentwhich takes the shape of a sinusoidal wave. Due to parasiticcapacitance, the sinusoidal waveform of the current may get shifted withrespect to the phase of the analog voltage signal. In someimplementations, the analog voltage and current signals can be convertedinto digital signals, and the lamp current can be identified from thedigital signals.

FIG. 8 is a flowchart of an example process 800 to identify a currentdrawn by a fluorescent lamp in a circuit. The process receives a voltagesupplied to a fluorescent lamp (805). For example, the processor, towhich the fluorescent lamp is operatively coupled, can be configured tocause a voltage source to supply a voltage signal, for example, ananalog voltage signal to the circuit including the lamp. The circuit caninclude a feedback loop operatively coupling the processor and thevoltage source. The analog voltage signal supplied to the circuitincluding the lamp can also be supplied to the processor via thefeedback loop.

The process digitally samples the voltage at a sampling frequency (810).For example, the circuit can include an analog-to-digital converter(ADC) to convert the analog voltage signal into a digital voltagesignal. The ADC can sample the analog voltage signal at a samplingfrequency. In sampling the analog voltage signal, the ADC can associatemultiple time stamps to a duration for which the analog voltage signalis received. The duration of the voltage signal depends on the frequencyand the operating cycle of the signal. For example, if the operatingcycle of the analog voltage signal is X ms, then the X ms are dividedinto multiple time stamps starting from 0 ms and ending at X ms in 0.01ms intervals. The interval between consecutive time stamps depends onthe sampling frequency of the ADC. The ADC can measure a value of thevoltage signal at each time stamp, and associate the value with thecorresponding time stamp.

The process associates a first time stamp with a voltage valuerepresenting an inflection (815). In some implementations, theinflection voltage can be the maximum voltage while, in otherimplementations, the inflection voltage can be the minimum voltage. Forexample, the processor can store the multiple voltage values that areassociated with the multiple time stamps. The processor can identify themaximum voltage value and the corresponding time stamp.

The process receives the current after receiving the voltage (820). Forexample, the feedback loop that transmits the analog voltage signal tothe processor can also be configured to transmit the current drawn bythe lamp to the processor.

The process digitally samples the current at the sampling frequency(825). The ADC can receive the current drawn by the lamp, and digitallysample the current at the ADC's sampling frequency in a manner similarto the digital sampling of the voltage signal.

The process associates multiple second time stamps with multiple currentvalues (830). For example, the processor can associate multiple timestamps and store multiple current values associated with the multipletime stamps.

The process identifies a second time stamp (835). The second time stampassociated with one of the current values corresponds to a first timestamp associated with the maximum voltage value. For example, theprocessor can compare the first time stamp and the second time stamp,and can determine that a difference between a first time stamp and thesecond time stamp lies within a threshold. In some implementations,because the time stamps for the current and voltage are generated at thesampling frequency, the multiple voltage time stamps are identical tothe multiple current time stamps. In such scenarios, the processoridentifies a second time stamp that is equal to the first time stamp. Insome implementations, although digital sampling is performed with thesame ADC, the multiple time stamps of current and voltage signals maynot be identical to each other. In such implementations, the processorchecks if a difference between corresponding time stamps is within athreshold. For example, the processor can determine a difference betweeneach time stamp on the voltage time scale and each corresponding timestamp on the current time scale, and can check if the difference is lessthan the threshold.

Subsequently, the process 800 identifies a current value associated withthe identified second time stamp (840). The current value associatedwith the identified time stamp represents a fluorescent lamp currentdistinguished from current due to parasitic capacitance. The currentthat is measured in the industrial lighting system 1000 is a combinationof fluorescent lamp current and parasitic capacitances from othercomponents in the system 1000. At 100% brightness levels, thecontribution of the parasitic capacitance is not significant incomparison to that of the fluorescent lamp current. When the brightnessof the lamp is decreased, for example, to or below 3% of the outputpower, the contribution of the parasitic capacitance increases. Theprocess 800 enables distinguishing between a lamp current and a currentdue to parasitic capacitance.

FIG. 9 is a schematic of an example circuit 900 to identify a currentdrawn by a fluorescent lamp in a circuit. The circuit 900 includes afluorescent lamp 905 to which an analog voltage signal is supplied usinga voltage source 910. In response, the fluorescent lamp 905 draws analternating current. The circuit 900 includes an ADC 915 that receivesthe analog voltage and the current. In some implementations, the ADC 915receives the voltage for multiple cycles. For each cycle, the ADC 915samples the voltage and associates time stamps with voltage values. TheADC 915 supplies the multiple time stamps and voltage values to theprocessor 920. The processor 920 identifies the maximum voltage value orthe minimum voltage value for each cycle. The processor 920 alsoidentifies the theoretical points of inflection of the voltage.

For example, the sampling time of the ADC 915 in the circuit 900 neednot be synchronized with the lamp voltage and/or lamp current waveforms.To identify the true lamp current value, the component of the currentthat is in-phase with the voltage needs to be identified. To do so, timestamps can be given to each measured sample of the voltage and the lamp.The time stamps' counter can be reset at each start of the cycle foreach of the parameters, namely for voltage, lamp current 1, and lampcurrent 2, in scenarios where two fluorescent lamps are included in thecircuit. Because the ADC sampling frequency is not synchronized with thelamp voltage and/or lamp current waveforms, the measurements can havedifferent time stamps for each sample of the measured parameter. Theprocessor 920 can identify the time stamp that corresponds to themaximum value of the voltage. Subsequently, the processor can perform asort action to identify the time stamp of the currents that is closestin value to the time stamp of the maximum value for the voltage. Thecurrent value that is associated with this identified time stamp istaken as the value in-phase with the lamp voltage. A similar process canbe applied to identify minimum values. In some implementations, thisprocess can be used for multiple cycles, for example, five cycles, toallow better identification of the maximum and minimum values.

In one example, the period of the measured signals (voltage andcurrents) is 10,200 ns, and the sampling period of the ADC is 500 ns. Ifthe ADC 915 collects samples every 500 ns, then the voltage waveform has20 samples (10,200/500) with the time stamps of the first cycle being500 ns, 1000 ns, 1500 ns, and so on. The second voltage cycle will havetime stamps of 300 ns, 800 ns, 1300 ns, and so on, the fourth voltagecycle will have time stamps of 100 ns, 600 ns, 1100 ns, and so on, thefourth voltage cycle will have time stamps at 400 ns, 900 ns, 1400 ns,and so on, and the fifth cycle will have time stamps at 0 ns, 500 ns,1000 ns, 1500 ns, and so on.

The first sampling is started at the time that the voltage measuringcycle is started and the voltage, lamp current 1, and lamp current 2 aremeasured in that sequence. If the voltage is a perfect sinus, then themaximum voltage value would be at 2550 ns and the minimum value would beat 7650 ns. For the maximum, sample cycles one and three, having timestamps at 2500 ns and 2600 ns will be equally distanced from the realmaximum, namely, 2550 ns. For the minimum, the third cycle will have thetime stamp closest to the real value, namely, the 7600 ns time stamp.The waveforms are not ideal. In some implementations, the ADC can have a10-bit resolution, and the sample voltages at 2500 ns and 2600 ns willnot be the same. A difference between the sample voltage at these twotime stamps can be compared to the theoretical maximum, and a voltagevalue that is nearest to the theoretical maximum can be identified.Based on this identification, either the 2500 ns or the 2600 ns timestamp can be chosen. In this example, it is assumed that the 2500 nsvoltage sample is the maximum.

Lamp current 1 will have the following time stamps:

-   Cycle 1—300 ns, 800 ns, 1300 ns, and so on.-   Cycle 2—100 ns, 600 ns, 1100 ns, and so on.-   Cycle 3—400 ns, 900 ns, 1400 ns, and so on.-   Cycle 4—0 ns, 500 ns, 1000 ns, 1500 ns, and so on.-   Cycle 5—300 ns, 800 ns, 1300 ns, and so on.

Lamp current 2 will have the following time stamps:

-   Cycle 1—100 ns, 600 ns, 1100 ns, and so on.-   Cycle 2—400 ns, 900 ns, 1400 ns, and so on.-   Cycle 3—0 ns, 500 ns, 1000 ns, 1500 ns, and so on.-   Cycle 4—300 ns, 800 ns, 1300 ns, and so on.-   Cycle 5—100 ns, 600 ns, 1100 ns, and so on.

After loading these values in the memory, the processor 920 chooses, forthe maximum lamp current 1, the sample associated with the 2500 ns timestamp, namely, from cycle 1. Similarly, for the minimum current, theprocessor 920 selects the sample associated with the 7600 ns time stamp,namely, from cycle 2. A similar procedure can be used for lamp 2 todetermine a maximum value as the current associated with the 2500 nstime stamp in cycle three, and the minimum value as the currentassociated with the 7600 ns time stamp in cycle 1. These values areconsidered the real values for the lamp current, even though themeasured waveforms may be different.

The ADC 915 digitally samples the received voltage and current signals.Thus, the processor 920 in the circuit 900 is configured to digitallyidentify a current drawn by a fluorescent lamp and to distinguish thefluorescent lamp current from a current generated by parasiticcapacitance. In addition, the processor 920 is configured to perform theoperations described with reference to FIG. 9.

If the lighting system is used to control lighting in a building, eachfluorescent lamp can be located in different parts of the building. Insuch scenarios, identifying which control signal from the processor issupplied to which lamp can be difficult, especially in the absence of ablueprint of the circuit. As an option, a technician can serially turnoff the control signals from the processor one signal at a time, andidentify that a given control signal is supplied to a given lamp whenthe lamp turns off. Another method to identify which control signal issupplied to which fluorescent lamp is related to RF signals offered by afluorescent lamp in response to receiving a control signal.

FIG. 10 is a flowchart of an example process 1100 to identify afluorescent lamp among multiple fluorescent lamps. The process receivesinput to identify which first control signal is provided to a firstfluorescent lamp (1005). For example, in an industrial lighting system,a circuit can include multiple fluorescent lamps operatively coupled toa voltage source that is controlled by a processor that causes thevoltage source to supply control signals to the multiple fluorescentlamps. The industrial lighting system can be employed in a building withseveral rooms, with each room including multiple fluorescent lamps. Theprocessor can be controlled by a central management facility thatcontrols the processor, thereby controlling the multiple fluorescentlamps in the building. The techniques described with reference to FIG.10 enable a technician to map control signals supplied by the voltagesource to fluorescent lamps that receive the control signals.

In response to the input, the process provides a first discovery signalin place of each control signal, one fluorescent lamp at a time (1010).For example, upon receiving input from the central management facility,the processor overrides control signals supplied to the fluorescentlamps with discovery signals, one fluorescent lamp at a time. Thediscovery signal can be chosen such that the discovery signal does notcause a change in lamp luminosity that is perceptible by the human eye.Further, the discovery signal causes a fluorescent lamp to which it issupplied to output electromagnetic radiation, for example, radiofrequency (RF) signals. In some implementation, this RF radiation isdetectable only in lamp's close vicinity. In this manner, the processortransmits a discovery signal to a first fluorescent lamp causing thefirst fluorescent lamp to output RF signals.

The process determines that the first fluorescent lamp outputs RFsignals (1015). For example, a technician can use an RF receiver todetermine whether the first fluorescent lamp outputs RF signals. Thetechnician can hold the RF receiver against the first fluorescent lampuntil the lamp emits the RF signals. The RF receiver can be configuredto transmit an indication that RF signals are emitted to the centralmanagement facility that controls the supply of control signals.Alternatively or in addition, the RF receiver can be coupled to adisplay device that displays such an indication.

The process identifies the control signal that was replaced with thediscovery signal that caused the first fluorescent lamp to output the RFsignals (1020). For example, the RF receiver transmits the indicationthat the first fluorescent lamp has outputted RF signals to the centralmanagement facility. The central management facility is aware of whichcontrol signal was overridden by a detection signal. Based on thisinformation, the central management facility maps the control signal tothe first fluorescent lamp, and stores the mapping in memory.Subsequently, the technician moves to the next fluorescent lamp, and theprocess 1100 is repeated.

In some implementations, the discovery signal sent to the multiplefluorescent lamps can have the same characteristics. This, in turn, cancause each fluorescent lamp to output similar RF signals. In someimplementations, each fluorescent lamp can be configured to outputunique RF signals in response to receiving the common discovery signals.In such implementations, the discovery signals can override all thecontrol signals simultaneously. As long as the RF signal emitted by onefluorescent lamp does not interfere with the RF signal emitted byanother fluorescent lamp, the RF signals of each lamp can be used toidentify each fluorescent lamp. For example, the fluorescent lampmanufacturer can include the type of RF signals output by the lamp aspart of lamp specifications. The central management facility can store amapping of a fluorescent lamp and the RF signal output by the lamp in amemory. The memory already includes a mapping of the fluorescent lampsand control signals. When the discovery signals are supplied to thefluorescent lamps, the RF signals output by each lamp can be measuredand compared against the mapping stored in the memory and used toidentify control signals that are supplied to the fluorescent lamps.

FIG. 11 is a schematic of an example circuit 1100 including multiplefluorescent lamps 1110 receiving control signals from a processor 1120controlled by a central management facility 1105. The processor 1120controls the supply of control signals to each fixture. The processor1120 receives input from the central management facility 1105 toidentify which control signal is supplied to which fluorescent lamp inthe circuit 1100. In response, the processor 1120 is configured toenable the technician to identify control signals supplied to thefixtures 1205 by performing operations described with reference to FIG.10.

In some implementations, the central management facility 1105 isconfigured to provide each fluorescent lamp 1110 in the circuit 1100with a unique RF signal that causes the fluorescent lamp 1110 thatreceives the discovery signal to output RF signals. The centralmanagement facility 1105 is operatively coupled to a memory 1115 thatstores a mapping of the unique detection signal and the correspondingunique RF signals. Based on the detected RF signals, the centralmanagement facility 1110 identifies the fluorescent lamp that issupplied with a unique detection signal. Subsequently, based on thecontrol signal that was replaced with the unique detection signal, thecentral management facility 1105 determines the detection signal that issupplied to the fluorescent lamp 1110. The unique detection signals canbe generated by modulating the control signal at a frequency. Thefrequency can be provided to the processor by the central managementfacility or can be stored in the processor.

In some implementations, the central management system can have storedin memory a map of rooms and positions of lamps. For example, each roomcan have an ID, and each lamp in the room can have an ID. Each lamp canbe driven by a corresponding processor, capable of emitting thediscovery signal. Each lamp fixture can have a unique name, made out of48 bit information. The central management system need not know aposition of a lamp and a fixture to which the lamp is attached. Thecentral management system can issue a discovery command causing thelamps to transmit their unique IDs, for example, as RF signals. As atechnician walks past each lamp, the technician detects the RF emittedby the lamp, for example, using an RF receiver. The RF receiver can belinked to a computer system on which is installed a software applicationconfigured to decode the RF signal and display the lamp's unique name.The technician assigns the lamp's name to the fixture in the room. Inthis manner, the technician identifies each lamp in the building andmaps the lamp to the corresponding fixture. In some implementations,having mapped a lamp to a fixture, the software application installed onthe technician's computer system can cause the central management systemto issue a command to turn off the lamp at the fixture. In response,when the lamp turns off, the technician can confirm the mapping.

FIG. 12 is a schematic of an example processor 1300 to control anindustrial lighting system. The processor 1200 is configured to performthe operations described with reference to FIGS. 2-12. The processor1200 can include the following components that perform the describedfunctions:

-   i. Internal Power Distributor—from the incoming V_(dd) builds and    distributes the necessary voltage and current for the inner blocks,-   ii. SYS Select Block—based on the values of two user selected    resistors placed at the two adjacent pins, it selects the working    mode for the device,-   iii. RXY LUT Select Block—based on the values of two user selected    resistors placed at the two adjacent pins, it selects the    Look-Up-Table wherefrom the device reads the working parameters, as    per the lamp types that it must drive,-   iv. Master Clock Generator 64 MHz Typical—self generating master    clock signal, used for the entire circuit,-   v. Clocks Management—distributes and controls various clock signals,    for various blocks of the device,-   vi. Timers Generators—under the state machine control, defines and    routes various timers,-   vii. 2 Wire Comms Interface—communication block, used for    communication between the state machine and the outside world,-   viii. State Machine Active Registers—register bank, with values    uploaded as per the chosen working mode,-   ix. LUT Regs Bank—Look-Up-Tables registers, with specific parameters    describing various lamp types,-   x. State Machine Processor—hard wired state machine. Logic processor    of incoming stimuli and generator of output timers and parameters,-   xi. Squarewaves generator—generates the three driving square waves,    as per the state machine instructions,-   xii. Level Shift and Amplifiers Block—takes the 7 input stimuli and    condition them as required for the internal process,-   xiii. Multiplexer 7 IN 1 OUT—multiplexing block, takes the 7 input    signal and time multiplex the output,-   xiv. ADC 10 bit 2 MHz—analog to digital Converter, 10 bit resolution    2 MHz sampling rate. It measures the signals outputted by the    Multiplexer,-   xv. Comparator block—compares the conditioned signals with voltage    thresholds and feed the information to the state machine,-   xvi. Voltage References—delivers the references for the thresholds    used by the Comparator block,-   xvii. Filament Driver, 3.3 V LVTTL—output driver for the Filament    control signal, 3.3V LVTTL capable,-   xviii. Top Driver, 3.3 V LVTTL—output driver for the Top Driver    control signal, 3.3V LVTTL capable, and-   xix. Bottom Driver, 3.3 V LVTTL—output driver for the Bottom Driver    control signal, 3.3V LVTTL capable.    The processor 1200 can be operatively coupled to an industrial    lighting system, for example, the lighting system 1000 shown in    FIG. 1. In some implementations, the processor 1200 can be    operatively coupled to a computer. A technician can provide input to    the processor 1200 through the computer. In response to the input,    the processor 1200 can control the industrial lighting system 1000.    In such implementations, the input to the processor 1200 can be    provided via one or more computer program products, tangibly    embodied on computer-readable media, to cause data processing    apparatus to perform operations to transfer the input provided by a    technician to the processor 1200. In some implementations, input to    the processor 1200 can be transmitted over one or more communication    networks.

A few embodiments have been described in detail above, and variousmodifications are possible. The disclosed subject matter, including thefunctional operations described in this specification, can beimplemented in electronic circuitry, computer hardware, firmware,software, or in combinations of them, such as the structural meansdisclosed in this specification and structural equivalents thereof,including potentially a program operable to cause one or more dataprocessing apparatus to perform the operations described (such as aprogram encoded in a computer-readable medium, which can be a memorydevice, a storage device, a machine-readable storage substrate, or otherphysical, machine-readable medium, or a combination of one or more ofthem).

The term “data processing apparatus” encompasses all apparatus, devices,and machines for processing data, including by way of example aprogrammable processor, a computer, or multiple processors or computers.The apparatus can include, in addition to hardware, code that creates anexecution environment for the computer program in question, e.g., codethat constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, or a combination of one or moreof them.

A program (also known as a computer program, software, softwareapplication, script, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, or declarative orprocedural languages, and it can be deployed in any form, including as astand alone program or as a module, component, subroutine, or other unitsuitable for use in a computing environment. A program does notnecessarily correspond to a file in a file system. A program can bestored in a portion of a file that holds other programs or data (e.g.,one or more scripts stored in a markup language document), in a singlefile dedicated to the program in question, or in multiple coordinatedfiles (e.g., files that store one or more modules, sub programs, orportions of code). A program can be deployed to be executed on onecomputer or on multiple computers that are located at one site ordistributed across multiple sites and interconnected by a communicationnetwork.

While this specification contains many specifics, these should not beconstrued as limitations on the scope of what may be claimed, but ratheras descriptions of features that may be specific to particularembodiments. Certain features that are described in this specificationin the context of separate embodiments can also be implemented incombination in a single embodiment. Conversely, various features thatare described in the context of a single embodiment can also beimplemented in multiple embodiments separately or in any suitablesubcombination. Moreover, although features may be described above asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can in some cases be excisedfrom the combination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments.

Other embodiments fall within the scope of the following claims. Forexample, the processor can be operatively coupled to the circuitsincluding the fluorescent lamp through one or more wireless networks,e.g., LANs. In this manner, a processor that is not coupled to a circuitby wired means can still be used to operate the fluorescent lamps.

1. A method comprising: receiving output of a circuit having at leasttwo loads and an anti-parallel transformer, wherein a first side of theanti-parallel transformer is operatively coupled to a first of theassociated loads such that a first current flowing through the firstload also flows through the first side of the anti-parallel transformer,and a second side of the anti-parallel transformer is operativelycoupled to a second of the associated loads such that a second currentflowing through the second load also flows through the second side ofthe anti-parallel transformer; monitoring, based on the output, aresistance of the anti-parallel transformer with respect to a thresholdcorresponding to the associated loads; and identifying that either thefirst of the associated loads or the second of the associated loads isabsent from the circuit responsive to themonitored resistance crossingthe threshold.
 2. The method of claim 1, wherein the at least two loadsis associated with a lamp and the lamp is a discharge lamp.
 3. Themethod of claim 2, wherein the lamp comprises two fluorescent lamps, thefirst load is one of two filaments in a first of the two fluorescentlamps, and the second load is one of two filaments in a second of thetwo fluorescent lamps.
 4. The method of claim 1, comprising: causing adetection voltage to be supplied to the circuit, responsive to theidentifying, at a frequency greater than an operating frequency usedwith the circuit, the detection voltage alternately turned on for afirst duration and turned off for a second duration; and measuring theresistance of the anti-parallel transformer when the detection voltageis supplied to the circuit.
 5. The method of claim 4, comprising causingthe detection voltage to be turned off, and an operating voltage to bere-supplied, responsive to the measured resistance being less than thethreshold.
 6. The method of claim 4, wherein the first duration is 10 msand the second duration is 300 ms.
 7. A non-transitory medium encoding aprogram operable to cause a processor to perform operations comprising:receiving output of a circuit having at least two loads and ananti-parallel transformer, wherein a first side of the anti-paralleltransformer is operatively coupled to a first of the associated loadssuch that a first current flowing through the first load also flowsthrough the first side of the anti-parallel transformer, and a secondside of the anti-parallel transformer is operatively coupled to a secondof the associated loads such that a second current flowing through thesecond load also flows through the second side of the anti-paralleltransformer; monitoring, based on the output, a resistance of theanti-parallel transformer with respect to a threshold corresponding tothe associated loads; and identifying that either the first of theassociated loads or the second of the associated loads is absent fromthe circuit responsive to the monitored resistance crossing thethreshold.
 8. The non-transitory medium of claim 7, wherein at least twoloads is associated with a lamp and the lamp is a discharge lamp.
 9. Thenon-transitory medium of claim 8, wherein the lamp comprises twofluorescent lamps, the first load is one of two filaments in a first ofthe two fluorescent lamps, and the second load is one of two filamentsin a second of the two fluorescent lamps.
 10. The non-transitory mediumof claim 7, the operations comprising: causing a detection voltage to besupplied to the circuit, responsive to the identifying, at a frequencygreater than an operating frequency used with the circuit, the detectionvoltage alternately turned on for a first duration and turned off for asecond duration; and measuring the resistance of the anti-paralleltransformer when the detection voltage is supplied to the circuit. 11.The non-transitory medium of claim 10, the operations comprising causingthe detection voltage to be turned off, and an operating voltage to bere-supplied, responsive to the measured resistance being less than thethreshold.
 12. The non-transitory medium of claim 10, wherein the firstduration is 10 ms and the second duration is 300 ms.
 13. A systemcomprising: a lamp having at least two associated loads; ananti-parallel transformer, wherein a first side of the anti-paralleltransformer is operatively coupled to a first of the associated loadssuch that a first current flowing through the first load also flowsthrough the first side of the anti-parallel transformer, and a secondside of the anti-parallel transformer is operatively coupled to a secondof the associated loads such that a second current flowing through thesecond load also flows through the second side of the anti-paralleltransformer; and a processor operatively coupled to receive outputassociated with the anti-parallel transformer, the processor configuredto perform operations comprising monitoring, based on the output, aresistance of the anti-parallel transformer with respect to a thresholdcorresponding to the associated loads, and identifying that either thefirst of the associated loads or the second of the associated loads isabsent responsive to the monitored resistance crossing the threshold.14. The system of claim 13, wherein the lamp is a discharge lamp. 15.The system of claim 14, wherein the lamp comprises two fluorescentlamps, the first load is one of two filaments in a first of the twofluorescent lamps, and the second load is one of two filaments in asecond of the two fluorescent lamps.
 16. The system of claim 13, theoperations comprising: causing a detection voltage to be supplied to thelamp, responsive to the identifying, at a frequency greater than anoperating frequency used with the lamp, the detection voltagealternately turned on for a first duration and turned off for a secondduration; and measuring the resistance of the anti-parallel transformerwhen the detection voltage is supplied to the lamp.
 17. The system ofclaim 16, the operations comprising causing the detection voltage to beturned off, and an operating voltage to be re-supplied, responsive tothe measured resistance being less than the threshold when the detectionvoltage is supplied to the lamp.
 18. The system of claim 16, wherein thefirst duration is 10 ms and the second duration is 300 ms.